A river the color of pale toffee coursed through a valley, carrying several types of rare fish. A young orangutan, a member of a threatened species, dangled merrily by one leg from a tree.

In the heart of Borneo, home to one of the world's last remaining expanses of intact rain forest, Hermas Rintik Maring, an avid conservationist who is native to the area, marveled at the life within the vast canopies of jungle green that for centuries have made this tropical island vital to the health of the region.

At the same time, he said, he fears this pristine forest could fall to the whine of chainsaws and the rumble of bulldozers clearing land for what has been billed as the world's largest palm oil plantation.

The project, brokered by the Indonesian government in Jakarta, could affect as many as 5 million acres of Borneo's forest -- an area slightly smaller than the state of Vermont -- near Indonesia's 1,250-mile-long border with Malaysia. Officials hope China will finance the project on the island, which is divided between Indonesia and Malaysia.

Indonesian officials claim the plantation could bring the area a half a million jobs directly related to the industry and 500,000 more in spin-off jobs in schools, health care and other services. It could produce more than 10 million tons of crude palm oil a year, they said, worth about $4.6 billion. Chinese officials said a project covering 5 million acres could cost up to $10 billion.

But environmentalists such as Hermas, 28, a field officer for the Worldwide Fund for Nature in Indonesia, worry that without careful planning the project could destroy Borneo's profusion of plants, insects and animals.

"It would be one of man's great mistakes," said Hermas, his eyes sweeping across a panorama of olive-colored forests and blue-gray mountains from a clearing 1,800 feet high. "It would be unforgivable."

The plan is still in its infancy. It envisions a series of large plantations owned by private companies and linked by roads and palm oil mills. Exactly where everything would go has not been decided. That lack of clarity has prompted growing controversy.

Palm oil, used in the age of the Egyptian pharaohs, is fast becoming one of the world's leading vegetable oils. The antioxidant-rich oil, squeezed from a reddish fruit about the size of a large plum, is used in products as diverse as chocolate, potato chips, detergent and lipstick. It is now being touted as a bio-fuel -- a clean alternative fuel -- as crude oil prices soar.

Malaysia, Indonesia's more prosperous neighbor to the north, is the world's number one producer. But if the project here proceeds as the government hopes, Indonesia will surpass it.

"Indonesia is lucky that God gave it a good place to build palm oil plantations," said Raden Pardede, a senior adviser to the Economic Affairs Ministry who is working on the project.

Earlier this year, President Susilo Bambang Yudhoyono visited East and West Kalimantan, two Indonesian provinces on Borneo that border the Malaysian part of the island. He met with governors and mayors, who appealed for roads, jobs and resources to combat rampant illegal logging.

Yudhoyono, a retired general and former security minister, has long wanted to bring investment to the border area. "Security and stability will be better," he told Tempo, an Indonesian newsmagazine, in an August interview. "Palm oil and agricultural cultivation will raise incomes, absorb the workforce and increase regional taxes. Meanwhile, we will be able to keep on nurturing the sense of nationhood and being Indonesian."

On a state visit to Beijing in July, he spoke to President Hu Jintao about helping to develop the border area. "So far, the feedback has been positive," Yudhoyono said.

Chinese officials are more circumspect. "We are proceeding very cautiously," a bank official in Beijing who is tracking the project said on condition of anonymity.

The Chinese said that though their government is generally keen to invest in Indonesia, especially in oil, natural gas, minerals and infrastructure, agricultural projects such as palm oil plantations require careful study. Environmental concerns, competing land claims and conflicting viewpoints of local governments could slow progress, said Tan Weiwen, economic and commercial counselor at the Chinese Embassy in Jakarta.

But, he said, he sees opportunity in a growing demand for palm oil in China, the world's third-largest importer of the commodity, and the world at large. "Where there is sugar," he said, "the ants will come."

Indonesian officials said they would conduct an in-depth feasibility study next year that will weigh environmental, social, economic and security costs and benefits. "Basically the idea is to increase the well-being of the people along the border," said Pardede, the adviser.

He said that he expected the project would safeguard wildlife. "We don't want to build in protected areas," he said.

He has heard the objections of environmentalists, who cited research showing that oil palms do not thrive above 670 feet. The forests near the border rise like cathedrals on mountains with elevations of 1,000 to 6,500 feet.

The area that environmentalists call the heart of Borneo is home to 14 of the island's 16 major rivers. Six miles downstream from Betung Kerihun is Danau Sentarum, a 325,000-acre necklace of lakes that nurture several species found only on Borneo, including the bekantan monkey and arwana fish. Indigenous peoples live and fish on the lakes. Logging the forests will start a chain reaction of erosion and silt buildup that will destroy the area's water ecosystem, environmentalists say.

At least one plan, drawn up by a consortium of state-owned palm oil plantations and obtained by the Worldwide Fund for Nature, shows plantations being built in three national parks near the border, including Betung Kerihun and Danau Sentarum.

Pardede dismissed that plan as unworkable. But environmentalists are taking no chances. Many local officials, they noted, are eager to see jobs coming their way.

The environmentalists are concerned that under the guise of planting oil palms, companies will raze the forests, removing billions of dollars' worth of timber, and then abandon the land. Too often over the last decade in Indonesia, that scenario has played out. According to the Forestry Ministry, 5.75 million acres of forest in Indonesian Borneo alone have been cleared for palm oil plantations that never materialized. Most of that land is at a lower altitude.

If the government wants to promote oil palm plantations, "Why not use that land?" asked Hermas, the conservationist.

On a recent day, Hermas motored four hours up the Embaloh River in a wooden longboat, past trees wearing fern garlands like feather boas. At a small clearing, he stepped lithely out of the boat, and he and several colleagues began to climb. With each step came a new vista. Sunlight dappled the forest floor, illuminating mushrooms, fern tendrils and giant ants skittering across twigs.

"This is orangutan habitat," he said, moving up steep slopes and whacking vines with a machete. He peered up at an abandoned orangutan nest, some 100 feet high in a tree. A decade ago, an estimated 100,000 orangutans frolicked in Borneo's forests; today there are only 55,000. Palm oil plantations are the main reason for the decline, according to Friends of the Earth, an international environmental group. The industry could drive the ape to extinction within 12 years, the group warned.

Hermas is one of Borneo's indigenous Dayaks. Dayaks do not know how to grow palm oil, which the Dutch introduced to Indonesia in the late 1800s. He worried that if plantations are built, workers from other parts of Indonesia will take most of the jobs, creating social conflicts.

Hermas was raised in a forest culture and now lives in a communal Dayak longhouse in a town that is hours away by boat and motorbike. On a recent walk, he paused to hear a peacock cooing. He noted dainty white orchids with lavender tips, a new species discovered several years ago. "The forest," he said, "is a genetic bank."

His parents trapped wild boar, he said. They showed him how to find edible rattan shoots. With his machete, he hacked a robust liana vine two inches in diameter, releasing a liquid that was cool and clear, like pure water. He held the bamboo-like tube to his mouth and took a deep swallow. "We can survive in the jungle," he said. "We can eat young leaves. We can get water from the liana. Everything can be eaten here."

This cultural heritage is at risk, he said.

"I, my grandparents, used to be in the jungle," he said. "We used many sources of the jungle. But if someday the jungle changes to palm oil, what can we do?"

A group of Indonesian environmentalists and reporters use longboats to enter a rain forest in Borneo that activists say is threatened by a development plan.A snake in Betung Kerihun National Park. Several species found in Borneo can be found nowhere else.Hermas Rintik Maring, a Borneo native, points out an edible rattan shoot in a rain forest on the island. He is critical of a proposed palm oil plantation.A small river courses through Betung Kerihun National Park. There are hundreds of rivers in the park, many of which feed into the larger Kapuas.